What effect would an exhaust leak have on engine performance? 

I think you might see a slight drop in performance.  I'd imagine the leak will cause more turbulance in your system and maybe force the ecu to make slight adjustments.

If its not a big leak, probably not the end of the world in the short term.

lots of popping on decel.

O.K.  I've easily got both of those.  I'm wondering if exhaust cans - cored slip-ons - could cause this?  Not enough back-pressure?  Would something like this cause a lean burning air/fuel problem.

If the PC wasn't richening things up... HOWEVER, slipons alone usually don't cause much of a change in a/f. I mean, they don't really do all that much at all, but make some extra sound. Full exhaust = different story. Open Airbox = different story.

Cored cans usually isn't a dramatic change.

Quote from: boobies on April 14, 2010, 02:21:56 PM
What effect would an exhaust leak have on engine performance? 

It would make lots of cool, loud noises - people would think you are going MUCH faster than you really are and admire you for your bravery and panache.

Sorry, didn't realize I was using a lot of technical jargon, but I hope this was helpful nonetheless.

George.  Tech Support in Dallas.

haha. better than tech support in India.

O, tank you very much... vood yu lyk a Slurpee or a lottaree tiket vit tat?

Exhaust leaks closer to the head/exhaust port are more prone to cause popping and backfiring - letting extra air mix with nice hot exhaust gases and unburnt fuel. 
Only if the leak was right at the header/exhaust port junction would you actually compromise the running of the motor, or.... if upstream of a working O2 sensor, it can also mess up sensor readings, causing poor running on closed loop systems.

Leaks further down stream  are more of annoyance. Plus they can make a sooty mess to clean up.  Which italian is the flatulent one?

It depends on where the leak is -and what you're doing...

If it's before the O2 sensor (if your engine has one) the Oxygen in the air mixing with the exhaust can cause the ECU to call for the wrong injector "on" time... (it sees too much Oxygen in the exhaust -so it changes the fuel/air mix to "correct" what it sees as a problem) causing backfiring and running/drivability issues, etc...

Otherwise (no O2 sensor) it just causes backfiring on decelleration, mostly.

On some air cooled engines I've worked on (Kohler V-twins specifically) I've seen cold air get sucked back into the cylinder through a crack in the exhaust, cool one side of the cylinder from the inside, cause it to contract into the piston, and wipe the whole cylinder/piston/ring assembly out.

if you would like to know what "real" popping sounds like, i can bring the S4R by sometime...  we're talking orville redenbacher meets m60 machine gun... plus some bona fide small "explosions".  and i have a fully stock intake.  and as the temps get warmer it just seems to become worse.

have you seen my exhaust? back pressure is overrated. i have exactly zero back pressure. i got rid of the "popping noise" with a some welding and a manipulator. was able to get the proper a/f mixture for a closed loop system. now it sounds like it did when it was stock ............except a little louder.

  "little"

These responses have helped answer several of my questions - I appreciate the feedback  

As far as which Italian is causing me the grief, it is, of course, the tractor.  If you ever fall out of love with your Duc someday, go ride a Guzzi.  Any Guzzi.  Crude blunt implement, meet buttery smooth laser prescision  

Stuart has agreed to look at the beast next week.  The more I talked with him and from the answers I've gotten here, I'm thinking the bike's issues are not minor.  

At any rate, the Guzzi is for sale.  Working on the craigslist ad now.

Thanks again for your support - y'all ROCK!

don't get another guzzi.  get the new multi if you need a comfortable touring bike with serious 'go' on tap. or shrug your shoulders and get a honda st. (sigh).